The present invention relates to a thermoplastic part with an injected elastomer seal, to a manufacturing process for a thermoplastic part with an injected elastomer seal, as well as to an injection molding apparatus for carrying out the process.
Thermoplastic parts with elastomer seals are used, for example, in the engine space of passenger cars or as housing of all type in the electrical industry. Oftentimes, it is required that these parts reliably retain their sealing function against water, oil etc. even after being dismantled for several times. The same is also true when exposed to extreme temperature stress. In view of their great elasticity over long periods and even at high temperatures (150xc2x0-200xc2x0 C.), seals of cross-linked plastics (e.g. rubber or LSR (liquid silicon rubber)) are very often used. Thermoplastic parts with seals of crosslinked plastics are, however, complicated and expensive to manufacture. The thermoplastic part is manufactured with a moderately heated mold (about 30-90xc2x0 C.), whereas the production of seals of crosslinked plastics requires the use of heated molds at temperatures of about 170-200xc2x0 C., so that it is a problem to manufacture both in a mold or in a fully automated process. Moreover, crosslinked plastics are very liquid during processing in the mold so that common sealing methods are insufficient in molds for thermoplasts. Thus, more or less overflowing during injection is experienced. As a consequence, the thermoplastic parts and the rubber seals or LSR seals are conventionally manufactured in two separate molds by different processes. The rubber seals or LSR seals are then typically placed into the grooves of the thermoplastic parts through cost-intensive manual work. Hereby, the placement by hand can be carried out in qualitatively very different manner, resulting in a fairly high reject fraction (up to about 15%).
Unlike crosslinked plastics, seals of thermoplastic elastomers can easily be injected in a fully automated way into the grooves of the thermoplastic parts. A drawback associated with thermoplastic elastomer seals is, however, their slight or significantly decreasing restoring capability. A repeated dismantling and assembly of thermoplastic parts with seals of thermoplastic elastomers is thus only possible to a very limited extent.
A further problem associated with seals of thermoplastic elastomers is the risk of detachment of the seal from the thermoplastic part during handling, when the seal does not have sufficient adhesion to the thermoplastic part.
It is thus an object of the invention, to provide a thermoplastic part which has an injected seal that is held reliably in the thermoplastic part and preferably retains its sealing properties even when the thermoplastic part is repeatedly dismantled and assembled as well as upon exposure to high temperatures.
A further object of the invention is the provision of a process, suitable for automation, for manufacturing such a thermoplastic part.
It is further an object of the invention to create an apparatus for carrying out this process.
The thermoplastic part according to the invention is characterized by mechanically anchoring the injected seal in the provided groove through one or more undercuts.
The process for manufacturing a thermoplastic part with injected seal from a crosslinked plastic includes a first step for manufacturing the thermoplastic part with a groove for receiving the seal, a second step for formation of undercuts, and a further step for injecting the elastomer into the groove. To prevent overflow during injection, sealing profiles may be formed on both sides of the groove on the thermoplastic part in the first step for sealing the groove during injection of the crosslinked plastic, or a mold can be used in the second step, having a core which includes sealing projections suited to the groove. Preferably, the sealing profiles and the sealing projections are pointed profiles of triangular cross section. When the sealing profiles are part of the thermoplastic part, they can be shaped in the direction of the interior of the groove either in the second step, during the closing of the shaping mold, so that undercuts are realized for holding the seal. When being arranged on the core, the sealing projections can be used to shape the edges of the groove toward the interior of the groove to thereby create the undercuts for anchoring the seal in the groove. Depending on the properties of the elastomer to be injected, the use of the sealing profiles and sealing projections may be omitted and the groove edges only are molded into undercuts.
The above-described process is carried out by an injection molding apparatus having at least one female mold and at least two cores, with the female mold and the first core defining a mold for injection molding the thermoplastic part, and the female mold and the second core defining a mold for injection molding the elastomer seal. Hereby, the second core can be heated entirely or partially, and either the first core is so configured that sealing profiles are molded on the thermoplastic part on both sides of the groove for sealing the groove in cooperation with the second mold during injection of the elastomer, or the second core has sealing projections which seal the groove on both sides during injection of the elastomer.
Preferably, the second core has a zone which is partly suited to the groove and is insulated from the remaining zones of the core and can be heated to about 170-230xc2x0 C. The heatable zone may also be implemented by an insert or several partial inserts which can be placed in the second core. The injection molding apparatus may include a device for automatically changing the cores. Preferably, the injection molding apparatus is a two-component machine.